Metal bonded abrasive composition



United States Patent Orifice 3,036,907 Patented May 29, 1962 Theinvention relates to metal bonds for abrasives and to metal bondedabrasive compositions for abrasive articles and the like. It finds itsbest present utility in the manufacture of diamond grinding wheels forthe grinding of cemented carbide tools, particularly for sharpeningmilling cutters and for the grinding of chip breaker grooves in cementedcarbide lathe tools. This application is a continuation-in-part of mycopending application Serial No. 797,641, filed March 6, 1959, and nowabandoned.

One object of the invention is to provide free cutting diamond grindingwheels which do not have excessive wheel wear. Another object is toeliminate or to reduce the use of abrasive dressing sticks becauseexcessive dressing causes loss of good abrasive which is particularlyundesirable in the case of expensive diamond abrasive. Another object isto provide wheels which do not crumble or spall yet which are freecutting.

Another object of the invention is to attain the desired freedom of cutin a diamond wheel Without the use of soft inert fillers which cause thewheel to break down too readily during grinding. Such wheels usuallyhave weak structures so that both diamond and filler are readily tornout causing the wheel to round and spall on the corners, necessitatingfrequent truing to preserve the desired contour of the wheel face and itis an object of my invention to reduce or to avoid this defect. Anotherobject is to provide a metal bond which is strong and yet capable ofbeing broken down under the stresses of use in a controlled manner, thusconstituting and eflicient bond in abrasive articles used for operationssuch as electrolytic grinding. Another object is to provide a metalbonded structure containing hard particulate inorganic material, havinguseful strength and wear resisting properties. Other objects will be inpart obvious or in part pointed out herein.

I have found that phosphorous compounds added to metal bonds forabrasive products cause desirable gradual breakdown of the bond duringuse. The result of the gradual breakdown is freedom of cut achieved withrelatively low wheel Wear. Microscopic study indicates that thephosphorous appears to go into the metal grain boundaries.

EXAMPLE I As an illustration of the way in which phosphorous compoundsenter the grain boundaries, I took 15 grams of electrolytic iron powderof minus 325 mesh size and 0.79 grams of iron(ous) phosphate in purifiedfinely divided precipitated powder form indicated to have the chemicalformula Fe (PO -8H O. This phosphate was calculated to contain 33.5% Feand 12.3% P. When heated in hydrogen, the ignited material wascalculated to contain 73.1% Fe and 26.9% P, which means that the ignitedmixture with iron powder contained about 0.6% P.

I mixed these dry powders together thoroughly by hand spatulation. Thetotal mixture was then pressed in a steel mold of rectangular cavityapproximately 1%" long x /2" wide to a pressure of 40 tons per sq. in.,producing a bar compact of 0.251" thickness. The bar was placed in acontrolled hydrogen atmosphere furnace and fired along with other testbars with a four hour soak at 800 C. The fired (sintered) bar wasmeasured to determine shrinkage and weighed to permit calculation ofsintered density. Rockwell hardness was determined and the bar wasbroken in cross-bending on 1" span with single point loading at adeformation rate of 0.25" per minute. Modulus of rupture was calculatedby the conventional beam formula which reduces to:

. 1.5 load in lb.

Rupt' lwidfiwfihickness) This is approximately an ASTM standard methodfor modulus of rupture tests of sintered metal compacts.

From half of the broken test bar, a metallographic sample was cut andmounted in Bakelite for polishing, etching and microscopic examinationin the conventional manner. The microstructure showed black linesconsidered to represent an iron-phosphorous compound, which X-raydiffraction examination indicated to be iron phosphide, segregated atthe iron grain boundaries, which boundaries were disclosed by furtheretching. A parallel experiment without any iron phosphate additionshowed none of these heavy black lines.

EXAMPLE II This illustrates the grain-by-grain breakdown that occurs inmy compositions when subjected to stresses such as would be caused bythe eroding action of grinding swarf in grinding use. As an incidentalfeature not believed to affect the breakdown characteristics, a smallsulfide additive was present for reasons to be subsequently discussed.Bars were made according to Example I except that I added powderedcopper sulfide in the amount of 5.0 weight percent of the total mixtureweight, with subsequent thorough mixing, follower by sintering.

A polished and etched microsection of this sample after having beenindented with a Rockwell hardness indenter showed a dark portion as thedepressed area produced by the indenter and it could readily be seenthat failure cracks had developed along the grain boundaries. Thesecracks are believed to have occurred along the grain boundaries whichwere weakened by the phosphorous containing compound.

A parallel experiment using electrolytic iron powder alone showed only anormal sintered ferritic structure and grain boundary weakening was notpresent. This was shown by a photomicrograph of a bar made and sinteredin an experiment exactly parallel to Example I except that nophosphorous compound was added. As in the preceding case, a dark portionwas the depressed area produced by a Rockdell hardness indentation. Inthe pure iron there was no evidence of grain-boundary failure and theproduct yielded in a ductile manner as would be expected forelectrolytic iron.

In my new bond a measure of ductility remains due to the highly ductilenature of the electrolytic iron in spite of the presence of phosphorousand resulting grain boundary weakening. This is proved by the fact thatI have data which shows that the deformation at rupture of typical barswas in the range of 20 to 40 mils, whereas that of similar bars made ofa typical brittle bronze bond as used in many diamond grinding wheels ofthe prior art is in the range of only 2 to 6 mils. This ductility isconsidered to account for the resistance to spalling and consequentcorner-holding ability of wheels made with my bond. It is to be realizedthat the inherent ductility of electrolytic iron powder prevails inspite of about 0.2% of residual impurities that it may contain.

As examples of the effect of different amounts of phosphorous compoundon the physical characteristics of metal bonds, I have made bars similarto Example 1 except that they were sintered at 815 C., in whichdifferent amounts of iron phosphate were used in the mixture. Testresults on 6 sintered bars of each mixture were as follows:

TABLE I Physical Test Results on Metal Bonds (av. of 6 bars) Composition(percent by Wt.) Density Hardness Modulus Example No. (gnL/ec.)(Rockwell of Rupture F-scalo) (psi) Total P lotal FezFe added as Fe plusFe added as iron phospirate Added as FQ2(P()4]2'8FI20 but calculated toFe plus P.

These results show how I can vary the amount of grainboundary weakeningby varying the amount of phosphorous in the bond. The crossbendingstrength decreases as I increase the amount of phosphorous in the bond.Since I have observed that phosphorous compounds enter the grainboundaries, I interpret the decreasing strength to be correlated with aprogressive weakening of the grain boundaries.

The following description of the making of a diamond wheel with ExampleIII bond illustrates the use of my new metal bond composition in agrinding wheel. The wheel was a straight wheel of 6" diameter by A5"thickness. The diamond-containing layer of the wheel was the peripheryand was A deep. I took 395 grams of electrolytic iron powder and moldeda preform for the center of the wheel by putting this powder in a steelmold 6.024" inside diameter with a 1.250" arbor positioned centrally andpressing it to a thickness of 0.202 inches which required a pressure ofabout 26 tons per square inch. Then the preform was turned in a lathe to5.889" diameter and replaced in the same mold band. The annular spacebetween the periphery of the preform and the band was filled with thediamond mixture made by thoroughly mixing 22.8 grams of Example III bondmixture with 2.10 grams of size #1008 diamond abrasive. The entire Wheelwas then pressed to a pressure of 40 tons per square inch. The mold wasthen stripped, the wheel removed, placed on a silicon carbide refractorybatt and put in a furnace for firing, which was done with a soakingtemperature of 815 C. for 4 hours in an atmosphere of hydrogen. Aftercooling to room temperature the wheel was trued to final dimensions andtested as will be described. Corresponding wheels with the other bondsof Table I are made in the manner just described. Wheels of other sizes,shapes, amount of diamond or other abrasive, grain size, kind of bondfiller can be made, and other variations can be introduced into my bondcomposition and used to make wheels with procedures well known in theart.

To demonstrate the practical advantages of my new product in grindingwheels, I ground with the Example III wheel in comparison with two otherwheels that are representative of current commercial wheels used forplunge cutting chip breaker grooves in cemented carbide tools. Agrinding test was devised for carrying out this type of operation understandardized conditions that could be accurately measured andreproduced. The wheels were all metal-bonded diamond wheels of about thesame grain size. They were 6" diameter and were sided to exactly 0.117"for all wheels. They were dressed by a standard method established forthe test before grinding. The results of the test are given in Table II.

4 TABLE I1 Grinding Results on Directly Comparable Metal-Bonded DiamondWheels Diametral i Wheel The above test shows superior performance forthe invention wheel in all three categories. The time required to carryout the amount of grinding established in the test was less, the wheelwear measured by the difference in mils between the diameter of thewheel before the test and after the test was less and the wear on thewheel corners measured by the average radius in mils of the two wheelcorners after the test was less.

In terms of the wheel value to the customer a lower grinding time meansgreater freedom of cut which means higher output of tools ground pershift with lower labor cost; a lower wheel wear means more tools groundper wheel and hence lower wheel cost; and a lower wheel corner wear addsto wheel life by reducing the amount of dressing necessary to hold thedesired contour of the tool.

As further illustration of the utility of my bond, the results of a fewfield tests that are representative of many such tests made with myproduct are as follows.

Plant A uses a metal-bonded diamond wheel, 6" x Ms", for plunge grindingthe cemented carbide flutes of a twofiute end mill. An :1 dilution ofsoluble oil in water is used as coolant. The test wheel made inaccordance with Example III bond was dressed at the start of the testand satisfactory wheel performance was noted throughout the test. Noglazing occurred at any time. The wheel corners held up better than anywheel ever used previously. No appreciable wheel wear had occurred aftergrinding 400 flutes. The radius at the corner was still less than andthe wheel had not been dressed since the test started. The customerwanted free cut and long life and he was getting both.

Plant B uses a 6" x Vs" diamond metal bond wheel for plunge grindingchip breakers on cemented carbide tools. The test wheel made inaccordance with Example IV bond ground 20,888 chip breakers, had asatisfactory action and indicated a reduced grinding cost compared tothe standard wheel used on the job.

Plant C uses a 3% x l /z" x 1%." flaring cup shaped resinoid-bondeddiamond wheel for resharpening cemented carbide inserts of millingcutters with soluble oil coolant. The test wheel made in accordance withExample IV bond was evaluated in an acceptance test by the customerconsisting of 0.100" stock removal on a carbide blank /2" x 1" using afeed of 0.0003" per pass. The customer found the test wheel to be thebest metal bonded cup wheel tested because of its cool and free-cuttingaction.

Plant D uses a 10" X 3& straight periphery-type diamond resinoid bondwheel for a combination of chip breaker grinding and surfacing ofcarbide tools with soluble oil coolant. The test wheel made inaccordance with Example III bond lasted for a total of 1,936 hours onproduction work, during which 25,600 tools were ground. This is the bestperformance ever obtaind from any diamond wheel and reduced the overalltool cost from $3.04 per hour to 10 cents per hour.

It is my theory that the phosphorous addition acts advantageousy notonly to establish a controlled breakdown rate in the bond by modifyingthe ferritic grain boundary but also in other ways. It reduces theductility of the iron, thus reducing the tendency of the wheel to smearin grinding. I also consider that it imparts an af- TABLE III Physical Tes! Results on Iron-Phosphorous Bond of Example III plus 5% 01'' AddedMetal Sulfide (av. of several bars) Example 8 331 Percent DensityHardness Modulus No. Ound in (gm/cc.) (Rockne-l1 or Rupture gddedProduct F Stale) (psi) FeS l 8 6.32 69 74. 000 C118 1 7 6. 28 77 86. 000M115 1 8 6. 22 71 By a photomicrograph at 400X of Example IX, darkergrayish rounded areas were noted and considered to be manganese sulfide.

It will be seen from Table III that these products made with sulfideshave a strength as measured by modulus of rupture that is not materiallyreduced compared with corresponding products made without sulfides,while the Rockwell hardness is slightly higher. I interpret theseeffects to means that smearing tendencies of the bond during grindingare reduced because of the sulfide inclusions and accompanying higherhardness, whereas wheel wear should remain essentially the same.

To investigate the effect of manganese sulfide addition in actualgrinding tests, I made a diamond abrasive wheel in accordance with theprocedures described for the making of Example III wheel except that asa bonding material I used the composition of Example IX, with MnS in thebond mixture as well as the 0.6% phosphorous. This wheel was used tosurface grind cemented tungsten carbide under fixed-feed conditions.Both wheel wear and power consumed were low and the performance wasconsidered generally satisfactory, whereas bronze-bonded wheels of theprior art used for this type operation consume excessive powerindicating failure to cut freely under surface grinding conditions.

To introduce phosphorus into my compositions I may use a varity ofphosphorous-containing materials. I have given the example of ferrousphosphate. Ferric phosphates, pyrophosphates, phosphites, phosphides andother compounds can be employed. Phosphorus compound concentrates at thegrain boundaries and weakens them. I may use phosphorous compounds otherthan those of iron, such as those of Ni, Co, Cu, Mn and CT. I may use aphosphorous content in my sintered bonds from about 0.4% to about 5% byweight of the total metal bond composition after the furnacing whichconverts the bond to a loss free ignited basis and develops strength bythe sintering operation, but I prefer bonds having in the range fromabout 0.6% to about 3% by weight of phosphorous of the total ignitedmetal bond composition.

To introduce sulphur into my compositions I may use metallic sulfideswhich are stable and do not dissociate or vaporize in the sinteringrange of my compositions. Sulfides of such metals as iron, nickel,cobalt, copper, manganese, chromium and mixtures thereof may be used.Sulphur compounds which are reducible in hydrogen, or other protectiveatmosphere used in sintering, to form sulfides may be used. The sulphurcontent of my bond composition may be from zero to 7% depending upon theoil-holding characteristics desired, although about 4% is usuallysulficient.

The sintering temperature can vary from about 750 C. to about 1100" C.,but the exact temperature, soaking time, kind of atmosphere and otherprocessing details will depend upon the composition used and otherprinciples well known in the sintering art. My metal bond is an ironbase bond which I define as containing at least 86% by weight of iron oriron strengthened with ferrite strengthening metal and having from nosignificant carbon up to .8% carbon and with from .4% to 5% phosphorousand with from no significant sulphur to 7% sulphur. However the ironbase bond has at least 50% of iron. Metals which strengthen ferrite aremanganese, silicon, nickel, cobalt, chromium, copper, molybdenum andtungsten. Mixtures can be used. My metal bond has a melting point above750 C. and is sintered at between that temperature and 1100 C. Thealloying metals can be present in the continuous iron phase in amountsthat form solid solutions with the iron. In the preferred form of myinvention by metal bond, which in most cases ought to be soft, has ahardness no greater than on the Rockwell F scale. But I believe thatuseful abrasive compositions can be made in accordance with myinvention, in which the ferrite strengthening metal is tungsten orsilicon, hav

ing a hardness considerably greater than 100 on this scale.

Another way of defining my invention which will serve to qualify theprevious definitions is that it is a metal bonded abrasive productconsisting of abrasive grain bonded with metal bond essentiallyconsisting of metal, phosphorous and permissible carbon and sulphur,having at least 50 iron, having at least 86% total metal, with from .4%to 5% of phosphorous, from no significant carbon up to .8% carbon, fromno significant sulphur up to 7% sulphur, said metal having a meltingpoint above 750 C. and having been sintered at a temperature of between750 C. and 1100 C., said phosphorous producing grain boundary weakening.

Furthermore in a preferred form of the invention, the iron isstrengthened by metal selected from Mn, Si, Nn, Co, Cr, Cu, Mo, W andmixtures thereof from .5% to the limit of solid solubility of such metalin iron. Also, my invention is a raw batch having the characteristicsand material contents in the percentages stated.

With regard to the solid solubility in iron of the various metals whichare ferrite strengthening metals, that of manganese is 3%, of silicon is14%, of nickel is 6%, of cobalt is 49%, of chromium is 25%, of copper is0.4%, of molybdenum is 6% and of tungsten is 6%.

The art of diamond wheel manufacture is now well developed and it isunnecessary to go into the details and permutations about such wheelsand their manufacture which can be embodied in the invention wheels ofthe examples without departing from the scope of the invention. Fordiamond abrasive articles the amount of diamond may be from about 5volume percent to about 38 volume percent of the article, whereas forordinary abrasives the amount may be from about 35 to about 75 volumepercent. The total range for all abrasives is therefore from about 5 toabout 75 volume percent of the article. Pores may also be present, butare usually low such as less than 20%, and often are substantiallyabsent.

It has become common practice in some types of metal bonded diamondwheels to use secondary abrasives or fillcrs in the metal bond, such asgranular or powdered tungsten carbide and other hard carbides. Othermaterials such as silicon carbide, aluminum oxide, etc., may also beused. Sometimes powdered glass, mica, etc., is used as a filler.

While for some purposes one bond will give the best results, for otherpurposes another, and for other purposes still another, as grindingrequirements vary, in order to comply with the statute, I select thebond of Example III as the best mode of the invention, and in the use ofdiamond grinding wheels, a wheel made with the bond of Example III.

It will thus be seen that there has been provided by this invention ametal bonded diamond abrasive composition in which the various objectshereinabove set forth together with many thoroughly practical advantagesare successfully achieved. As many possible embodiments may be made ofthe above invention and as many changes might be made in the embodimentsabove set forth, it is to be understood that all matter hereinbefore setforth is to be interpreted as illustrative and not in a limiting sense.Although I have explained my results in terms of observations and theirinterpretation by theories which represent my best opinions of processesand mechanisms which at best can only be interpreted and not completelymeasured, my claims are not restricted to the absolute correctness ofthese theories and opinions.

I claim:

1. A metal bonded abrasive product consisting of abrasive grain of from5 to 75 volume percent bonded with metal bond consisting essentially ofat least 86% by weight of metal selected from the group consisting ofiron and ferrite strengthening metals, and mixtures thereof,

at least 50% of the weight of the bond being iron, the bond alsoincluding from 0.4% to 5% by weight phosphorous, from a trace amount ofcarbon to 0.8% carbon, and from zero to 7% sulphur, said bond having amelting point above 750 C. and having been sintered at a temperature ofbetween 750 C. and 1100" C., said phosphorous producing grain boundaryweakening.

2. A metal bonded abrasive product according to claim 1 in which theferrite strengthening metal is selected from the group consisting of Mn,Si. Ni, Co, Cr, Cu, Mo, W and mixtures thereof from 0.5% to the limit ofsolid solubility of such metal in iron.

3. A metal bonded product according ot claim 1 in which the metal bondcontains from 1.5% to 7% sulphur.

References Cited in the file of this patent UNITED STATES PATENTS2,670,281 Hutchinson Feb. 23, 1954 2,895,816 Cline July 21, 1959 FOREIGNPATENTS 616,901 Great Britain Ian. 28, 1949 667,016 Great Britain Feb.20, 1952

1. A METAL BONDED ABRASIVE PRODUCT CONSISTING OF ABRASIVE GRAIN OF FROM5 TO 75 VOLUME PERCENT BONDED WITH METAL BOND CONSISTING ESSENTIALLY OFAT LEAST 86% BY WEIGHT OF METAL SELECTED FROM THE GROUP CONSISTING OFIRON AND FERRITE STRENGTHENING METALS, AND MIXTURES THEREOF, AT LEAST50% OF THE WEIGHT OF THE BOND BEING IRON, THE BOND ALSO INCLUDING FROM0.4% TO 5% BY WEIGHT PHOSPHOROUS, FROM A TRACE AMOUNT OF CARBON TO 0.8%CARBON AND FROM ZERO TO 7% SULPHUR, SAID BOND HAVING A MELTING POINTABOVE 750*C. AND HAVING BEEN SINTERED AT A TEMPERATURE OF BETWEEN 750*C.AND 1100*C., SAID PHOSPHOROUS PRODUCING GRAIN BOUNDARY WEAKENING
 2. AMETAL BONDED ABRASIVE PRODUCT ACCORDING TO CLAIM 1 IN WHICH THE FERRITESTRENGTHENING METAL IS SELECTED FROM THE GROUP CONSISTING OF MN, SI, NI,CO, CR, CU,MO,W AND MIXTURES THEREOF FROM 0.5% TO THE LIMIT OF SOLIDSOLUBILITY OF SUCH METAL IN IRON.